This invention relates to a method of manufacturing an optical waveguide which is for use in guiding an optical ray or a light wave and is applicable, for example, to an optical switch, an optical distributor, and an optical combinator, and the like. This invention relates, in particular, to a method of quickly diffusing an optical passage in the optical waveguide.
As a rule, an optical waveguide of the type described is used for transmitting a light wave in a transmission device so as to carry out optical communication with the other devices. The optical waveguide comprises a transparent substrate and a light-transmitting passage or an optical passage diffused in the transparent substrate. In this event, the transparent substrate is formed by a material containing a primary ion, such as sodium ion (Na.sup.+), which provides a predetermined refractive index. On the other hand, the optical passage is formed by a material containing a secondary ionizable species, such as silver (Ag), which provides a refractive index greater than the predetermined refractive index of the primary ion.
With this structure, the optical waveguide can guide the light wave through the optical passage without leakage of the light wave because the optical passage has a refractive index greater than that of the transparent substrate.
Heretofore, a conventional method is described in an article (page 93, 28a-A-2 fall, 1982) of Extended Abstracts which are published on Conference of Applied Physics. This conventional method may be referred to as a dry method. In this method, the optical passage is diffused into the transparent substrate by impressing a d.c. voltage in a dry atmosphere.
More particularly, the transparent substrate of, for example, glass is prepared which has primary and secondary surfaces and which contains a primary ion, for example, Na ion providing a predetermined refractive index. Thereafter, a diffusion-suppressing layer of, for example, titanium (Ti) is covered on the primary surface of the transparent substrate and is partially removed by the use of a photolithographic technique to selectively expose the primary surface and to thereby define an exposed area of the primary surface. The diffusion-suppressing layer may be operable as a diffusion-preventing layer or as a mask. Subsequently, an overlying layer is deposited by the use of sputtering or vacuum evaporation on the exposed area and the diffusion-suppressing layer. The overlying layer contains a secondary ionizable species of, for example, Ag providing a refractive index greater than the predetermined refractive index.
Finally, the d.c. voltage is impressed between the overlying layer and the secondary surface of the transparent substrate to ionize the secondary ionizable species, to diffuse or migrate secondary ions into the transparent substrate, and to thereby form the optical passage diffused in the transparent substrate. Such impression of the d.c. voltage is carried out after electrodes are deposited on the overlying layer and the secondary surface by the use of sputtering or vacuum evaporation.
However, it is difficult to quickly diffuse the secondary ions into the substrate from the overlying layer, because the primary ions are diffused in the same direction as the secondary ions towards the secondary surface of the substrate and thereafter accumulated near the secondary surface of the substrate without being discharged. In consequence, no electric current is caused to flow through the substrate due to such occurrence of the accumulation of the primary ions. Thus, the conventional method is disadvantageous in that it takes a long time to diffuse an optical passage in the substrate and to thereby form an optical waveguide.